Internal combustion engines are generally provided with fuel injection systems in which the pressurizing of the fuel and the control of injection are separated. In such systems an accumulator volume is used as a storage of pressurized fuel the admission of which into a combustion chamber is accomplished by means of a valve needle in the fuel injector nozzle. These systems are commonly known as common rail fuel injection systems.
The pressure and quantity of the fuel supplied into the nozzle has also significance for the quantity and quality of the combustion gases generated by the combustion process, which gases eventually load the environment. Therefore, the aim has been to control the fuel flow rate and pressure within the feed line by means of various, relatively complicated, electronic systems.
In common rail system the control of the valve needle is typically accomplished by making use of the fuel pressure acting against a surface of the needle in a direction of opening and/or closing the needle. In a typical common rail system the injection pressure reaches a high pressure level almost immediately when the valve needle starts rising in the injector nozzle.
Consequently, fuel is injected into the combustion space so that the mass flow is extremely voluminous from the very start of the injection. In this case, the cylinder pressure may rise too fast for achieving an optimum performance. Thus, the maximum pressure shown by the injection pressure curve (the pressure in the nozzle at various moments of time during the injection process) is generally reached too early. In addition, the reduction of the injection pressure takes time before the start of the next injection.
In order to control the rate of the increase of the pressure it is known to utilize so-called rate shaping of fuel injection by means of a rate shaping device.
EP 1686257 A2 discloses fuel injector in which the valve needle is controlled by means of subjecting the fuel pressure against the needle surface both into the opening and the closing direction at which situation the needle is maintained closed. In the event of injection the fuel pressure acting against the needle surface in the closing direction is relieved by opening a spill port in the nozzle, which causes a change in the balance of forces and thus movement of the needle. EP 1686257 A2 discloses also a rate shaping system which restricts the fuel flow during the initial portion of an injection event and causes a greater flow of fuel during a later portion of the injection event. In this arrangement the rate shaping is based on changing the restrictions provided in a main fuel line leading fuel to a sac of the injector.
Even if this fuel injector may be fit to use as such there has emerged a need for more precise control of the rate shaping functionality. Therefore it is an object of the invention is to provide a fuel injection apparatus for a piston engine which operates better in various operation conditions of the engine.